1. Field of Invention
This invention relates generally to battery chargers. More specifically, the invention relates to battery chargers of the non-constant-current type.
2. Description of Related Art
It is known to use storage batteries to power electromotive tools. Such tools are used in a repetitive fashion and the storage batteries are cycled often through their charge and discharge cycle. It is also known to rapidly charge such a storage battery by flowing a large current through it. Battery chargers of this type are known. Such battery chargers detect a fully-charged state of a storage battery pack by various arrangements.
One such known arrangement uses a thermostat which is installed in a storage battery pack for detecting the full charge. When charging is complete, charging current causes the battery pack to heat. When the temperature reaches a predetermined point, the thermostat opens to interrupt charging. A problem associated with the operation of this type of battery charger is that the thermostat sometimes breaks down at the moment when terminals of the storage battery pack short-circuit.
When the battery charger cannot detect the heat of the storage battery pack due to a failure of the thermostat of the battery pack, continued charging current may cause the storage battery to break down. Therefore even charging systems of type that use a thermostat also use a thermal-detecting device, such as a thermal-fuse in the battery charger as a safety device.
When a storage battery pack including a plurality of storage batteries is used repeatedly, one or more of the storage batteries may eventually short-circuit. When a short-circuit occurs in the storage battery, a battery charger that is not of the constant-current type flows a large current, causing damage to the battery charger. Therefore, an element for detecting the over-current, such as a current fuse or a hand-operated reversion type over-current relay is installed in the battery charger.
Thus, known battery charges of the non-constant current type often have separate elements for detecting a temperature rise due to over-charging and for detecting over-current. This arrangement is complicated to construct. When a current fuse is used as the element for protecting against over-current, it is necessary to keep extra fuses on hand to change them each time it becomes necessary. When a hand-operated reversion type over-current relay is used, it is necessary for an operator to reset a reversion switch whenever the relay operates.